1. Field of the Invention
This invention relates to an organic electroluminescent device.
2. Background Art
An organic electroluminescent device (hereinafter “organic EL device”) has been studied extensively for providing high-brightness luminescence at a low driving voltage. An organic electroluminescent device has an organic light emitting layer sandwiched between a pair of opposing electrodes. With an electric field applied to the electrodes, electrons and holes are injected from the cathode and the anode, respectively, into the organic layer, where they are recombined to generate excitons, the energy of which is utilized for light emission.
Recently, the luminescence efficiency of devices has been increasing by the use of a phosphorescent material. Phosphorescent electroluminescent devices using an iridium complex or a platinum complex as a phosphorescent material are disclosed, e.g., in U.S. Pat. No. 6,303,238 and WO 00/57676. Devices achieving both high efficiency and high durability, however, have not yet been developed. One of the reasons for the difficulty in developing a phosphorescent device with high efficiency and high durability is that there are limited host materials that have good chemical stability, excellent carrier injection/transport properties, and a high energy level of the lowest excited triplet state (T1 energy). A host material is required to have a higher T1 than a phosphorescent material because, and if not, the luminescence is quenched. When the T1 of a host material is higher than, but with only a small difference from, that of a phosphorescent material, back energy transfer from the phosphorescent material to the host material occurs partially, which causes a reduction in efficiency or durability. Therefore, a host material having a sufficiently high T1, good chemical stability, and excellent carrier injection/transport properties.
An organic electroluminescent device having a tetraphenylsilane compound of the following formula as a host material forming a light emitting layer along with a phosphorescent material is disclosed (U.S. Patent Publication No. 2004/0209116). Although the T1 energy level of the host material proposed is high, the charge injection and transfer properties are low so that the device needs an increased driving voltage. Improvements in this respect have been demanded.

JP-4-126790A describes an organic electroluminescent device having an m-phenylenediamine compound of the following formula.

The above compound was turned out by the present inventors' study to have a low T1 energy level. When it is used with a phosphorescent material, the compound quenches the luminescence of the phosphorescent material, only to achieve low luminescence efficiency and insufficient driving durability.